The present invention relates generally to package elevating wrapping machines and, more particularly, to a collapsible elevator for use in such machines to reduce the possibilities of jams and damage which can occur as a package is being elevated to be wrapped by the machines.
A large variety of package wrapping machines are known in the prior art. One variety which is presently very popular in the supermarket industry includes a package elevator. In these machines, packages comprising trayed products, such as meat, produce or the like, are placed into the machine at an infeed station, and conveyed from the infeed station to a wrapping station including the elevator which then raises the packages into sheets of wrapping material. The sheets of wrapping material are folded about the packages and secured therebeneath for attactive display of the contents of the packages. An illustrative package elevating wrapping machine is disclosed in U.S. Pat. No. 4,513,558 which issued to Fritz F. Treiber on Apr. 30, 1985, is entitled "Jam Detection and Removal for Wrapping Machine", and is incorporated herein by reference.
Unfortunately, in package elevating wrapping machines, it is possible for packages to become jammed within portions of the machine. In particular, packages may become jammed between the elevator and parts of the film folding apparatus due to misadjustment of the machine or misalignment of the packages on the elevator. Since the wrapping machines are typically large and driven by powerful motors, package jams can damage the machine and also present difficult and time consuming removal problems for the machine operator.
In the prior art, jam conditions are dealt with mechanically by means of torque limiters, lever assemblies, the slippage of drive belts and the like. in the referenced Treiber patent, the wrapping machine is controlled by a computer in accordance with clock counts generated by the main drive shaft of the machine. A jam is detected by counting the number of computer operating cycles which occur during a given clock count. If the number of cycles exceeds a defined number, a jam condition is indicated since such an overflow indicates that the main drive shaft is not moving at normal operating speed and, hence, not changing the clock count.
While the prior art reduces the forces which are brought to bear on packages which are jammed in package elevating wrapping machines, the machines must still be able to exert substantial forces to accomplish wrapping operations on packages weighing up to several pounds. Accordingly, the jams which still occur, even though early detected and of reduced force, can lead to damage of the wrapping machine sufficient to require a service call. This is particularly true in the case of jams involving the elevator which must exert substantial force in synchronism and close cooperation with the relatively more susceptible film underfolding apparatus. Hence, while the severity of a jam can be controlled to some extent by known prior art devices and jams detected to shut down the machine, such jams can still result in substantial damage and time consuming jam removal operations, particularly with relation to the elevator in package elevating machines.
It is thus apparent that the need exists for an improved jam detection and limiting arrangement for the elevator of a package elevating wrapping machine to prevent substantial damage to the machine upon the occurrence of elevator related package jams yet permit the machine to operate at its optimum to produce attractive, tightly wrapped packages.